another squealing champ

[dwhutchens]

My son has this little Yamaha practice amp POS so I decided to put some tubes in it. I went with a champ AA764 schematic. I used a cheap openframe 250vac PT with a bridge rectifier to save space. I had a Weber W022921 that I thought was a Champ OT/Reverb driver. My B+ is 366, screen is 358 but my plate is only 311vdc. I think this transformer is only a reverb driver and not suitable for the 6v6 OT use. It actually sounds pretty good but breaks into ocillation if I go past 7 or 8 on the volume with the treble maxed or vice versa. I have a real silverface Champ and it has a cap across from the grid to cathode of the 6v6 that's not on the schematic. Could the OT be causing the ocillations or do I need to add the extra cap, if so what would be the common value? This is my first single ended build.
Thanks,
David

[Twist]

Sounds like you need to make sure your grid wires are shielded properly. It may be suffering from oscillating at a high frequency. Try the same value cap that the Champ uses.

[dwhutchens]

I used shielded wire from the volume pot to pin 7 of the 12ax7 grounded on the pot end only. I forgot to mention that the Yamaha amp had a mastervolume control so I put a pot in that hole in place of the 220k from the 6v6 grid to ground. I also used shielded wire from the wiper to the 6v6 grid. I also have the neg feedback resistor disconnected. those are the only changes to the circuit I made. The cap is a small ceramic disk with what I think is the number 330 on it the best I can make it out, what value would that indicate?

[dwhutchens]

Oops, I noticed another difference. I have the 12ax7 backwards. Pins 1, 2 & 3 are facing the 6v6. Do I need to rotate the socket 180 degrees (pita) or can I just switch pins 123 with 678 or does it really matter?

[dwhutchens]

Ok, I flip floped the wiring on the 12ax7 and I think the squealing is under control. I switched the master volume pot over to the champs normal volume control and put the gain pot between the input jack and the first grid. Now I just have to chase some 120hz buzz.